It's been known for more than two centuries that pneumonia cases increase during flu epidemics.

But population-level epidemiological studies looking at seasonal patterns of influenza and pneumococcal pneumonia incidence have revealed either a modest association or have failed to identify any signature of interaction between the two.

These seemingly inconsistent observations at the personal and population scales have puzzled public health officials. Now Professor Pej Rohani and his colleagues have used a novel approach that they say resolves the dichotomy and shows that influenza infection increases susceptibility to pneumococcus, the most common bacterial cause of pneumonia, by about 100-fold.

An accurate characterization of the influenza-pneumococcal interaction can lead to more effective clinical care and public health measures, including influenza pandemic preparedness, according to the authors.

"The results concerning the nature of the interaction between influenza and pneumococcal pneumonia were unequivocal in our study," said U-M population ecologist and epidemiologist Rohani, senior author of a paper published online in Science Translational Medicine June 26, 2013. "Simply put, our analyses identified a short-lived but significant—about 100-fold—increase in the risk of pneumococcal pneumonia following influenza infection."

Rohani said the results suggest that the best way to reduce the incidence of bacterial pneumonia is to encourage the public to receive both pneumococcal and influenza vaccinations.

Rohani is a professor of ecology and evolutionary biology, a professor of complex systems and a professor of epidemiology at the School of Public Health. The first author of the Science Translational Medicine paper is Sourya Shrestha, a former postdoctoral fellow in Rohani's laboratory.